Abstract:
Since the beginning of the 2000s, several authors reported the occurrence of As and S as chemical components of the hydrated copper hydroxy-carbonate claraite, originally described with the formula (Cu,Zn)3(CO3)(OH)4·4H2O. Owing to the lack of knowledge about the crystal structure of this mineral, the structural role played by these chemical elements was unknown. The crystal structure of claraite has now been solved from single-crystal X-ray diffraction data by using a specimen from the marble quarries of Carrara, Apuan Alps, Tuscany, Italy. Electron-microprobe analyses gave (in wt% – average of eight spot analyses): SO3 4.00, As2O5 13.16, CuO 52.64, ZnO 9.03, CO2(calc) 9.08, H2O(calc) 12.56, total 100.47. On the basis of 15 (Cu + Zn) and 45 O atoms per formula unit, the chemical formula of claraite could be written as (Cu12.85Zn2.15)Σ15.00(AsO4)2.22(CO3)4(SO4)0.97(OH)13.40·6.83H2O, ideally (Cu,Zn)15(AsO4)2(CO3)4(SO4)(OH)14·7H2O. Raman spectrometry shows bands related to bending and stretching vibrations of AsO4 and SO4 groups, as well as the stretching mode of CO3 groups and O–H bonds. The unit-cell parameters of claraite are a = 10.3343(6)Å, b = 12.8212(7)Å, c = 14.7889(9)Å, α = 113.196(4)°, β = 90.811(4)°, γ = 89.818(4)°, V = 1800.9(2) Å3, space group Embedded Image . The crystal structure has been refined to R1 = 0.111 on the basis of 6956 reflections with Fo > 4σ(Fo) and 363 refined parameters. Claraite shows a layered structure, with {001} heteropolyhedral layers formed by Cuϕ5 and Cuϕ6 polyhedra as well as AsO4 and CO3 groups. These layers are stacked along c through edge-sharing Cu2ϕ10 and Cu2ϕ8 dimers, the former being decorated by corner-sharing SO4 groups hosted within intra-framework channels together with H2O groups. Claraite is the only known mineral showing the simultaneous occurrence of essential AsO4, CO3, and SO4 groups.

Marco E. Ciriotti

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